That's assuming it'll be a glorious victory with most of the Alliance and rebel Horde forces intact. For all we know it may end up being a pyrrhic victory, where we just barely manage to defeat whatever Sha/Demonic forces Garrosh has conjured. That would solve the problem by making it so the remnants of the Alliance army would be in no position to occupy Orgrimmar, and the remnants of the Horde forces would be left to pick up the pieces.

I can't even imagine the rage and butthurt that will be out there if the Sin'dorei manage to take over the Horde. I would love it, of course, and would go back to SMC to watch the Blood Elf guy giving the tour trying to advertise the Blood Elves to the other Horde races and thinking "you had this in mind from the start, didn't you?"